Generally, fine particles are defined to have a size smaller than 100 nm. Metal oxide particles having such a size have been widely used in the polishing fields including wafer-polishing CMP (Chemical Mechanical Polishing) slurry, cosmetics/medicine fields including UV protective cream, chemical reaction catalyst fields, and other fields including coating, magnetic devices, electronic devices and optical devices. In the future, the fine metal oxide particles will be expected to act as an important factor for next-generation nano-technologies, environmental/energy technologies, bio-technologies, etc.
Examples of the preparation methods of the fine metal oxide particles include a gas phase method, a liquid phase method and a solid phase method, according to the reaction phase.
The gas phase method comprises the steps of vaporization of a metal or metal precursor followed by a reaction of the vaporized metal or metal precursor with oxygen. In such a case, according to the types of the vaporization and the reaction, the gas phase method is classified into flame combustion pyrolysis, laser vaporization, plasma vaporization, spray pyrolysis, etc. The gas phase method is advantageous in terms of the simplicity, and the uniformity and fineness of particles, but suffers from high energy consumption, an expensive device and low productivity, thus negating economic benefits.
Further, the solid phase method is exemplified by a firing and a mechnochemical synthesis. In particular, the firing method is regarded as a typical preparation of inorganic particles, in which a precursor is subjected to pyrolysis for a long time in a furnace maintained at a high temperature, and then oxidized, to produce a metal oxide, which is then crystallized for another long time and pulverized to fine particles. The firing method involves the simple preparation processes, but is disadvantageous in terms of easy incorporation of impurities, and the reaction at high temperatures for long periods. Meanwhile, the mechanochemical synthesis method is characterized in that the surface of the metal precursor is activated by mechanical impetus of high energy and high speed. However, this method has disadvantages, such as the incorporation of impurities caused by abrasion of balls and a vial during a milling process, the extended reaction time and the requirement of a calcining process.
In addition, the liquid phase method includes hydrothermal synthesis, sol-gel method, micro-emulsion method and the like. As for the micro-emulsion method, a mixture of surfactant and metal precursor is reacted in a micelle so that the resulting product is precipitated, whereby it is possible to produce the particles having uniform sizes. However, the micelle serving as a reactor has a low concentration, and thus wastes may be generated in larger amounts. As well, the productivity is low, and also, there are other disadvantages, such as the use of expensive surfactants and the use of the calcining process. Also, the sol-gel method is mainly used for the preparation of TiO2, by which uniform and fine particles can be prepared. But, it is difficult to realize mass production. In the hydrothermal synthesis widely used as the liquid phase method, water is used as a reaction medium or a reactant. As such, the employed temperature and pressure are not too high. However, the produced particles have a large particle size and a wide particle size distribution. In particular, when the raw materials are nitrates, sulfates and hydrochlorides, waste acids are generated as in the other liquid phase methods.
As a kind of the hydrothermal synthesis, a supercritical hydrothermal synthesis method is reviewed in Ind. Eng. Chem. Res. Vol. 39, 4901–4907 (2000) by Ajiri et. al, Japan. According to the above method, a water-soluble metal salt is reacted under supercritical water conditions (temperature ≧374° C., pressure ≧221.2 bar), to easily produce nano-sized particles. However, also the supercritical hydrothermal synthesis method suffers from the production of a waste acid as represented by the following Reaction 1:
Reaction 1M(NO3)x+xH2O=M(OH)x+xHNO3(by-product)M(OH)x=MOx/2+x/2H2O
Wherein, M denotes a metal.
As for the supercritical hydrothermal synthesis process, a batch-type and a continuous-type are disclosed in WO 87/04421 and U.S. Pat. No. 5,635,154, respectively. In the batch-type, the reaction is performed for relatively longer periods, i.e., tens of minutes, and thus it is difficult to control the particle size, and a wide particle size distribution is obtained. Whereas, the continuous-type reaction is performed within a short time of 0.1 second to a few minutes, whereby there is no calcining process and the resulting product is very pure. In addition, it is easy to control the crystallization and the crystal size.
In U.S. Pat. Nos. 5,433,878, 5,480,630 and 5,635,154, there is disclosed a method of producing fine metal oxide particles by decomposition-reacting a metal salt at 200° C. or higher under 250–500 kg/cm2 for 1–10 min by use of a continuous tube reactor. However, the above method still has drawbacks, including the production of waste acids such as nitric acid and hydrochloric acid.
At present, the amount of by-product, inter alia harmful nitrogen-containing compounds (e.g., nitric acid), generated from the above preparation process have been strictly restricted based on environmental laws. Accordingly, a treatment for converting the harmful nitrogen-containing compound is additionally required. In these situations, research on techniques of treating the produced nitrogen compounds has been vigorously performed.
In this regard, Japanese Patent Laid-open Publication No. 2001–197449 discloses that a nitrogen-containing compound and an oxidizing agent are subjected to a hydrothermal reaction under supercritical or subcritical conditions, and a catalyst-inhibiting material is removed from the reactant, after which the generated ammonia or N2O is decomposed in the presence of a specific catalyst. Further, in Ind. Eng. Chem. Res. Vol. 37, pp. 2547–2557 (1997), there is disclosed the decomposition of nitrate and ammonia under supercritical water conditions of a temperature of about 450–530° C. and a pressure of about 300 bar. In addition, the oxidation of nitrogen-containing wastewater under supercritical water conditions results in generating the nitrogenation reaction between the ammonium ion and the nitrate ion, which is reported in J. Korean Ind. Eng. Chem. Vol. 11, No. 4, pp. 432–438. Also, with the intention of decomposing the generated nitrogen compounds, various methods, such as biological decomposition or catalyst decomposition, are proposed. However, techniques for preparing the fine metal oxide particles and for treating the harmful nitrogen-containing compounds produced concurrently with the synthesis of the fine metal oxide particles in the same reactor have not yet been developed.